Mammography is widely used to provide a two-dimensional image of breast tissue for breast examinations in diagnosing cancer or other abnormalities. The breasts, which have a delicate three-dimensional structure, are radiographed using a special X-ray apparatus.
Typical X-ray apparatuses use intense (hard) X-rays to expose bones and other hard objects. On the other hand, X-ray apparatuses used for breast examinations handle soft tissue such as fat and mammary glands, and thus use weak (soft) X-rays. At the same time, since the breast examination is the only examination in which skin comes into direct contact with a machine of the X-ray apparatus, various precautions are taken.
The mammography apparatus takes radiographs by pulling and compressing the breast. Radiographs are taken, for example, in two directions: in MLO (mediolateral-oblique) and CC (cranial-caudal) views.
When a radiographic image is initially displayed, a conventional mammography apparatus finds a maximum pixel value and a minimum pixel value in the radiographic image and set a center pixel value between the maximum pixel value and the minimum pixel value as a window level. The apparatus performs this calculation automatically (auto window function) and presents a display image based on the radiographic image. Also, regardless of the presence or absence of an implant (silicon pack) in the breast, the auto window function of the conventional mammography apparatus calculates a cumulative frequency beginning with the minimum pixel value using pixel values of the radiographic image and sets the pixel value at which the cumulative frequency reaches a predetermined threshold (e.g., 10% of the total) as the minimum pixel value.
The auto window function is disclosed by, for example, a medical diagnostic imaging apparatus which automatically calculates a maximum pixel value, minimum pixel value, and window level appropriate for a region which needs to be examined.
However, with the auto window function of the conventional mammography apparatus, if a radiographic image contains an implant part (a portion showing an implant), a proportion of the implant part in the radiographic image is large and pixel values of the implant part are relatively small. Thus, a histogram of a radiographic image containing the implant part has a peak on a low-pixel-value side, causing a cumulative frequency calculated starting from the low-pixel-value side to shift greatly to the low-pixel-value side when compared to a histogram of a radiographic image not containing the implant part. Consequently, with the auto window function of the conventional mammography apparatus, the display image based on a radiographic image becomes generally dark, resulting in a window in which the mammary gland and the like are difficult to observe.